In preparation of removing the legacy Xen driver, move the
sexpr2xml tests from WITH_XEN to WITH_LIBXL. Even though the
legacy driver will be removed, we'll want to maintain the ability
to convert sexpr to XML. Requires fixing up the tests to account
for different behavior of Xen vs libxl post parse functions.

There is some test file fallout due to differences in handling
of default values between xend and libxl.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>